Theory Thursday

Bike Sizing

So, you’ve got your helmet sorted, but what if you haven’t got a bike in the shed, or you want to upgrade or change from your old hybrid/commuter/mountain bike to a road bike for triathlon racing?  You need to buy a bike!

do you need a new bike??

You’ll want to set a budget.  Starting at £200 and going up to over £5000, road bikes are a big outlay.  The more you spend, the lighter the bike (easier uphill!) and the better the components.  But, in the end, the bike is only as good as the rider!  We also think, the bike is only as good as the fit.  If you end up spending long hours in the saddle on a 100 mile sportive, you don’t want to be cramped or overstretched.  Equally, if you want to go fast, efficient “power transfer” depends on having the right size (of frame) and fit (saddle and handlebar position, crank arm length)

Off the peg frame-sizing

If you are a normal-sized adult then there’s no problem in buying “off the peg”.  You will want to know your inside leg measurement and your total height.  You might need to add your

“ape index” – arm span minus height.

Every manufacturer and brand and frame design has a slightly different “geometry”, and you should be able to research this online, within your chosen budget, before even approaching the bike shop.  Generally, the manufacturer will recommend frame size (based on the length of the top tube) by your height.  

But then you need to go into the bike geometry for that frame size and check the “standover height”  This is where your inside leg comes into play.  ideally you want 3-5cm clearance in height over the top tube to avoid uncomfortable groin squashage when you stop. (potential ouch!)  

Then, if you find you sit between two frame sizes your “ape index” becomes important.  If it’s positive (big wings) go for the larger frame, if negative (short arms relative to height) go for the smaller frame.

Extremes of height

If you’re a hobbit, women’s specific road bikes are not only for women!  They are simply designed for a smaller frame with narrower shoulders and longer legs relative to shorter torso.  You can find the smallest frame size of 44cm in a few different manufacturers, but they are not universal.  If you’re a giant, you are fairly well catered for.

women’s specific bike – not in pink!


But if you are a narrow-shouldered extremely tall person or a broad-shouldered hobbit, and you have money to burn, you might want a custom build.  This is where you will find a bike shop with a fitting equipment (We recommend “Retul” as its an infinitely adjustable bike you actually sit on) to find the perfect geometry for you.  This means you might get less high spec. components/frame material within your budget, as you’re spending more on the custom build, but it may be worth your while to get ultimate comfort.

Invest to save

Finally, invest your time in finding the right bike size and then getting it fitted properly (saddle height etc. in the shop BEFORE you take it home) and it will repay you.  Big chain bike shops are only as good as their local staff, so if you decide to go to one don’t settle for any superficial fitting, go prepared with a few options and try before you buy.  Smaller shops tend to be run by bike enthusiasts and, by contrast, you may struggle to ever leave as you get engrossed in discussions about power output ratios……..!


Published by


A tiny hobbit dedicated to saving the world with a decent second breakfast. Also, available to make you feel good in triathlon when you overtake her.......

Leave a Reply

Your email address will not be published. Required fields are marked *